Regulation of muscle is studied at the molecular level in vertebrate and invertebrate muscles. Interaction of calcium with the control proteins that trigger contraction, in particular the mechanism of control exerted by myosin, is explored in detail. This regulation is mediated by a light chain component of myosin that is required both for the calcium dependence of ATPase activity, of tension development and for calcium binding. The regulatory light chains can be reversibly removed from scallop myosin without denaturing the heavy chains. Hybrids can also be formed between the heavy chains of scallop and the light chains of various myosins including both vertebrate and invertebrate muscles. The light chains of myosins that have a functional myosin control will be compared with those where myosin control is lacking. The interaction of the heavy chain and of the regulatory light chains will be analyzed using biochemical, immunological and structural approaches.